The combination of a lens and a detector array is commonly used to give an image of a scene on the array. The scene is imaged on the detector array and the angular extent of the scene that can be imaged is limited by the angular aperture of the lens that gives adequate resolution, and the size of the detector array. Radiation entering more obliquely does not normally fall on the detector array directly and indeed for such rays the imaging capability of the lens may be impaired. This is illustrated in FIG. 1 which shows rays from a distant scene falling on a lens 1 and, possibly after passing through a filter 4, form an image on the detector array 2 placed in the focal plane of the lens. Rays such as AA′ and BB′ give rise to elements of an image of acceptable quality and the detector array may therefore be placed so that A is illuminated by such rays. More oblique rays such as CC′ give rise to a distorted image of unacceptable quality for the accuracy of imaging required and fall outside the detector array. Rays incident at angles which give rise to acceptable images are said to be within the normal angular aperture of the lens. More oblique illumination is usually prevented from falling on the array by stops and by blacking the interior of the equipment. An arrangement is described which utilises these oblique rays so that the normal operation of the array is enhanced by some sensitivity to more oblique illumination. These oblique rays could of course be used in combination with a larger detector array covering the area where rays such as CC′ approximately converge, but the additional expense of employing a larger detector array and a larger aperture filter may be undesirable. The arrangement to be described can be used to enhance sensitivity, albeit without imaging, outside the portion of the scene which is normally imaged. No such arrangements are required where individual detectors are used to monitor events without imaging, as the angular aperture, for example that determined by a window, can be varied at will, and the question of image quality does not arise.
One application of the invention arises in the testing of equipment embodying a lens and detector array. Often in conjunction with a window. Where detectors are in continuous use to monitor events, particularly those required to indicate a fault or alarm condition. it is desirable to test frequently the continued operation of the detectors and the cleanliness of any window fixed in its housing. For systems where the monitoring uses single detectors with no focusing optics, arrangements are known where this monitoring is accomplished by mounting within the detector housing a subsidiary source of radiation within the wavelength range where the detector is sensitive. Radiation is prevented from falling directly on the detector by a suitable shield but emerges through a portion of the window and is reflected back through another portion of the window on to the detector. If the window is blocked or the detector has ceased to operate no signal due to the internal source is received. Partial obscuration or imperfect operation of the detector will result in a smaller signal. A class of such arrangements is described in U.S. Pat. No. 3,952,196 (1976). The incorporation of the source within the housing which can be sealed not only is convenient mechanically but the seal prevents the source itself from coming into contact with the air externally and constituting an additional risk of fire or explosion. However where an array of detectors is used with a lens to focus an image of the scene on to it, it is desirable to test not only that the window remains clean but also whether each element of the array continues to operate. For this purpose it is necessary to irradiate all the elements (but not necessarily simultaneously) of the detector array from the test source through the window whilst shielding the elements from direct exposure to the source of radiation. As a lens is used to focus the radiation from the external scene on to the detector array simple arrangements that are used with single detectors cannot be used.